In an intensity image, each pixel responds with a change in voltage of current due to arrival of light at a given wavelength on the focal plane, e.g., no voltage for no light impinging the pixel, some voltage (for some light) or maximum voltage (for maximum intensity). Associating a color or degree of black with the reported voltages forms an image. For example, the low light pixels become black, the pixels with some light become gray, and the pixels with excessive light appear white.
In a polarimetric image, each pixel also provides the polarization state of the incident light incident on that pixel. The polarization state is given by the intensity plus three additional values indicating the fraction of the light that was polarized, the axis of polarization, and whether the polarization is rotating in time. Polarimetric imaging thus provides more information than intensity imaging.
Polarimetric imaging is particularly useful in discerning man-made objects because they typically have a higher degree of polarization, hereinafter referred to as “DOP,” than natural objects. Polarization information also provides information regarding the surface roughness and orientation of an object that is not readily available from an intensity image. It has also been used to advantage in remote sensing, automatic target recognition, identifying materials, and distinguishing objects from a cluttered background.
Polarimetric data is often represented in terms of the four Stokes parameters, S0, S1, S2, and S3. These parameters represent all the polarization information, and are defined as follows:
 S0=I0+I90,S1=I0−I90,S2I45−I135, andS3=IR−IL
where Ix is the measured intensity of the light after passing through a linear filter at an orientation of X degrees, and
IR and IL are the measured intensities of the right or left circularly polarized fraction of the light.
The DOP, in terms of Stokes parameters, is given byDOP=√{square root over (S12+S22+S32)}/S0
Imaging polarimeters of the prior art typically collect several different images of the same object, with the light emanating from the object passing through a different polarization filter in each image. A variation described in U.S. Pat. No. 5,045,701 uses a rotating quarter-wave plate with a linear filter. With a single focal plane, this device takes several images at different rotations, respectively, of the polarizing filter. The collected images are averaged and subtracted to extract the four Stokes parameters at each pixel. To perform the subtraction, one registers pixels from the different images that represent the same point in the scene. However, because the images are taken at different times, any motion in the image will cause a registration error. This precludes the use of this apparatus to obtain polarimetric images of terrestrial objects from moving platforms, such as airplanes or orbiting satellites, or images of objects that are translating or rotating with respect to the apparatus.
Another approach uses four separate cameras with a different polarization filter on each camera. The four cameras take simultaneous images of the same scene. Again, the appropriate images are added and subtracted to extract the four Stokes parameters; however, parallax and camera misalignment will introduce registration errors into the derived image.
A third approach to polarization detection capitalizes on the polarization-dependent absorption of quantum wells, and is described by D. W. Beekman and J. Van And a, “Polarization Sensitive QWIP Thermal Imager,” Infrared Physics and Technology, Vol. 42, pp. 323-328 (2001). As will subsequently be discussed in detail, a quantum well with a linear grating can detect the component of incident light with the electric field perpendicular to the grooves of the grating. On a single focal plane, one makes neighboring pixels sensitive to vertical, horizontal or diagonal polarizations. Again, one adds or subtracts the images taken by the pixels sensitive to the different polarizations. Because the pixels being added and subtracted for each polarization image spatially neighboring points in the scene, sharp edges or bright points register as erroneous polarization.
The final example is called a polarimetric spectral intensity modulation spectropolarimeter, and is described in U.S. Pat. No. 6,490,043. This device measures the polarization of a single point in a scene by modulating the spectrum of the light with the polarization of the light and then measuring the spectrum of the light. In order to find the polarization one must compare the modulated spectrum to the true spectrum. Because the true spectrum is not known, approximations must be made that necessarily sacrifice polarimetric and spectral accuracy and precision in favor of pixel registration.
As shown by the foregoing discussion, there is a need in the art for a polarimeter capable of providing polarized images of an object that is translating or rotating relative to the position or orientation of the focal plane without sacrificing spectral or polarimetric accuracy and precision. The present invention fulfills this need in the art.